Image processing method, apparatus and controller

ABSTRACT

An image forming apparatus and method in which a controller is capable of executing image processing of its own are disclosed. The controller has an RF unit for processing image information, a halftone image processor, a density corrector and a patch generator. The engine includes a modulator corresponding to the RF unit, a halftone image processor different from that of the controller, a density controller, modulator and patch generator corresponding to the halftone processor of the engine, a sensor for sensing the density of formed patches, and a control unit for controlling the density of an image on the basis of a signal from the sensor. The engine sends the controller a signal relating to density control.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of Ser. No. 08/989,683,filed Dec. 12, 1997, issued as U.S. Pat. No. 6,111,664 which is acontinuation of application Ser. No. 08/426,275, filed Apr. 21, 1995,abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an image processing apparatus, method andcontroller for controlling density.

2. Description of the Related Art

FIG. 18 is a block diagram illustrating the construction of a printerapparatus according to the prior art. The printer proper, indicated at202, is connected to a host computer (hereinafter referred to as a“host”), which is an information source of image information to beprinted.

The printer 202 comprises a controller 206 and an engine 207, whichreceives print information outputted by the controller 206 and printsthe data on recording paper. The controller 206 comprises an interfaceI/F 203 for connection to the host 201, a frame buffer 204 for retainingdata, which has been transferred from the host 201, as image data to beprinted, and a reproduction unit (hereinafter referred to as an “RFunit”) 205, which subjects the output of the frame buffer 204 to maskingand UCR processing to convert the output to a signal suited to theengine 207.

The printer 202 constructed as described above will now be described.Multivalued image data that has entered from the host 201 is applied toand retained in the frame buffer 204 via the interface 203. At the timeof printing, the image data is read out of the frame buffer 204 in syncwith the recording speed of the engine 207. The image data read out isconverted by the RF unit 205 in conformity with the characteristics ofthe engine 207. By way of example, if the image input data has enteredin the form of an RGB signal, the data is converted to a signal capableof being handled by the engine 207, namely YMCK four-color image data ina case where full-color printing is performed using the colors Y, M, C,K. At this time masking processing or UCR processing, which has beenoptimized for the process characteristics of the engine 207, namely thetoner characteristics and development bias—density characteristic, isexecuted. The engine 207 prints out the image data that has undergonethe aforesaid processing.

In this multivalued image recording apparatus, it is required that therelationship between the entering density-level signal and the densityactually printed be linear, and it is necessary that the density printedbe constant with respect to the same density-level signal regardless oftemperature and humidity. In an electrophotographic printer, however,fluctuations in the toner characteristic and development bias—densitycharacteristic make it difficult to maintain density linearity andconsistency. In general, automatic density control for each of thecolors Y, M, C, K is carried out in the engine.

The following problems are encountered in the prior art described above:

In the arrangement described above, the color conversion characteristicsof masking and UCR in the RF unit 205 of the controller 206 must haveone-to-one correspondence with the process characteristics of the engine207. Consequently, when the controller 206 is developed, for example,the start of development must wait for settlement of the engine processcharacteristics, as a result of which the development period isprolonged. Furthermore, image processing functions inclusive of a colorconverting function cannot be added onto the controller, and thereforethe manufactured product cannot be provided with additional value.

More specifically, in the engine 207 which has a constructionindependent of that of the controller 206, the density of the printimage is fixed at a density characteristic which seems to be ideal.Accordingly, image processing carried out in the controller 206 beforedata is outputted to the engine 207 is not faithfully reflected in thefinal output image, and tones or colors that satisfy various user needssatisfactorily cannot be expressed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image processingapparatus, method and controller in which the aforementioned drawbacksare eliminated.

Specifically, an object of the present invention is to generate optimumcorrection data by a controller unit.

According to the present invention, the foregoing object is attained byproviding a controller unit for subjecting image data to a firstcorrection on the basis of first correction data and transmitting thecorrected image data to an image forming unit, the controller unitcomprising bidirectional communication means for bidirectionalcommunication with the image forming unit, and first generating meansfor generating the first correction data on the basis of datarepresenting first patches, formed by the image forming unit, obtainedby the bidirectional communication.

Further, an object of the present invention is to provide an imageprocessing apparatus having a degree of freedom high enough to meetvarious user needs relating to image processing.

According to the present invention, the foregoing object is attained byproviding an image processing apparatus having a controller unit forconverting entered image information to a multivalued image signal, andan engine for forming an image on the basis of the multivalued imagesignal, wherein the controller unit includes image processing means forprocessing the image information and first density-control processingmeans for performing first density control processing automatically, andthe engine includes image forming means for forming an image on thebasis of the multivalued image signal and second density-controlprocessing means for performing second density control processingautomatically.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of the arrangement ofa controller and engine in an image forming apparatus embodying thepresent invention;

FIG. 2 is a block diagram showing a case in which the engine of FIG. 1is evaluated;

FIG. 3 is a block diagram illustrating an example of the detailedarrangement of a halftone image processor, a density correcting unit anda modulator in FIG. 1;

FIG. 4 is a waveform diagram showing an example of the output of acounter;

FIG. 5 is a diagram for describing patch formation on a transfer drum inFIG. 1;

FIG. 6 is a graph showing an example of a density characteristicobtained by measuring a patch pattern;

FIG. 7 is a block diagram illustrating an example of a detailedarrangement of a halftone image processor, a density correcting unit anda modulator in FIG. 1;

FIG. 8 is a waveform diagram showing an example of the output of acounter;

FIG. 9 is a block diagram showing an example of an arrangement accordingto a modification of the present invention;

FIG. 10 is a block diagram showing an example of an arrangementaccording to a modification of the present invention;

FIG. 11 is a block diagram showing an example of an arrangementaccording to a modification of the present invention;

FIG. 12 is diagram showing an example of formed patches;

FIG. 13 is a sectional view showing an example of an image processingapparatus according to the present invention;

FIG. 14 is a block diagram showing an example of an arrangementaccording to a second embodiment;

FIG. 15 is a diagram showing an example of operation in density controlprocessing;

FIG. 16 is a block diagram showing an example of an arrangementaccording to a modification;

FIG. 17 is a diagram showing an example of formed patches;

FIG. 18 is a diagram showing the construction of a printer according tothe prior art; and

FIG. 19 is a diagram illustrating functions of a CPU, according to anembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

First Embodiment

FIG. 13 illustrates the construction of a color LBP (laser-beam printer)according to an embodiment of the present invention. As shown in FIG.13, transfer paper P fed from a paper feeder 101 is held on the outerperiphery of a transfer drum 103 via a conveyance path 102 while itsleading edge is gripped by a gripper 103 f. Latent images formed for therespective colors on a photosensitive drum 100 by an optical unit 117are developed by respective color developing units Dy, Dc, Db, Dm andare transferred a plurality of times to the recording paper on the outerperiphery of the transfer drum 103, whereby a multicolored image isformed. Thereafter, the transfer paper P is peeled off the transfer drum103 by a separating finger 113, the transfer paper undergoes fixing in afixing unit 104 and the fixed transfer paper is discharged into adischarge tray 106 via a paper discharge unit 105 or discharged into adischarge tray 115.

Each of the color developing units Dy, Dc, Db, Dm has a rotary shaftsupported at both ends. The developing units are held in adeveloping-unit selecting mechanism 108 so as to be capable of beingrotated about their respective axes. The color developing units Dy, Dc,Db, Dm may be rotated, so that any one can be selected, while theattitude thereof is held fixed. The selected developing unit is moved toa developing position, after which the developing-unit selectingmechanism 108 is moved, together with a selecting-mechanism supportingframe 109, toward the photosensitive drum 100 about a fulcrum 109 b bymeans of a solenoid 109 a.

The operation of the color LBP constructed as set forth above will bedescribed next.

First, the photosensitive drum 100 is uniformly charged to a prescribedpolarity by a corona discharge unit 111, and the drum is exposed to alaser beam L to develop the latent image of, say, the color M (magenta)on the photosensitive drum 100, thereby forming a first toner image ofthe color M (magenta) on the drum 100. Meanwhile, the transfer paper Pis supplied at a prescribed timing, a transfer bias voltage (e.g., +1.8kV) having a polarity (positive, for example) opposite that of the toneris applied to the transfer drum 103, the first toner image on thephotosensitive drum 100 is transferred to the transfer paper P and thetransfer paper P is then electrostatically attracted to the surface ofthe transfer drum 103. The photosensitive drum 100 then has any residualtoner of the color M removed by a cleaner 112 in order to prepare forthe formation of the next latent image and the next development process.

Next, the second latent image, this time for the color C (cyan), isformed on the photosensitive drum 100 by the laser beam L, then thesecond latent image on the photosensitive drum 100 is developed by thedeveloping unit Dc for C (cyan) to form a second toner image. The secondtoner image of the color C (cyan) is transferred to the transfer paper Pat a position matching that of the first toner image of the color M(magenta). In this second transfer of the toner image, a bias voltagehigher than that used at the time of the transfer of the first tonerimage is applied to the transfer drum 103 immediately before thetransfer paper arrives at the transfer section. Similarly, third andfourth latent images for the colors Y (yellow) and K (black) are formedon the photosensitive drum 100 one after the other, these latent imagesare developed successively by the developing units Dy, Db, respectively,and third and fourth toner images of the colors Y (yellow) and K (black)are transferred one after the other to the transfer paper P at positionsmatching those of the toner images transferred earlier. Toner images offour colors are thus formed in a superimposed state. In these third andfourth transfers of the toner image, a bias voltage higher than thatused at the time of the transfer of the second toner image is applied tothe transfer drum 103 immediately before the transfer paper arrives atthe transfer section.

The reason for thus raising the transfer bias voltage whenever thetransfer of the image of each color is performed is to prevent a declinein the transfer efficiency. The chief cause of a decline in transferefficiency is as follows: When the transfer paper is peeled off thephotosensitive drum 100 after the transfer, the surface of the transferpaper is charged by gaseous discharge to a polarity opposite that of thetransfer bias voltage (the surface of the transfer drum bearing thetransfer paper also is charges slightly). Since the electric chargeproduced by this charging accumulates with every transfer, the transferelectric field declines with every transfer if the transfer bias voltageis kept fixed.

This embodiment is so adapted that when the leading edge of the transferpaper arrives at the transfer starting position (this includes a momentimmediately before or immediately after arrival) in the fourth colortransfer mentioned above, a DC bias voltage of the same polarity andpotential as those of the transfer voltage applied at the time of thetransfer of the fourth toner image is superimposed upon an AC voltageand the resulting voltage is impressed upon the corona discharge unit111, thereby discharging the photosensitive drum 100. The reason forthus operating the corona discharge unit 111 when the leading edge ofthe transfer paper has arrived at the transfer starting position in thefourth color transfer is to prevent uneven transfer. In particular, intransfer of a full-color image, even the occurrence of slight transferunevenness results in a conspicuous disparity in color. Accordingly, itis required that a discharge operation be performed by applying thenecessary bias voltage to the corona discharge unit 111 in the mannerset forth above.

When the leading edge of the transfer paper P to which the toner imageof the four colors have been transferred in superimposed form approachesthe peel-off position, the separation finger 113 is made to approachthis position and the tip of the finger is brought into contact with thesurface of the transfer drum 103 to separate the transfer paper P fromthe transfer drum 103. The tip of the separation finger 113 is held incontact with the surface of the transfer drum and the finger is thenseparated from the transfer drum 103 and returned to its originalposition. The corona discharge unit 111 operates from the moment theleading edge of the transfer paper arrives at the final transferstarting position to the moment the trailing edge of the transfer paperdeparts from the transfer drum 103 and the accumulated charge on thetransfer paper (which charge has a polarity opposite that of the toner)is removed to facilitate separation of the transfer paper by theseparation finger 113 and to reduce the gaseous discharge at the time ofseparation. Furthermore, when the trailing edge of the transfer paperarrives at the transfer end position (the exit of a nipping portionformed by the photosensitive drum 100 and transfer drum 103), thetransfer bias voltage applied to the transfer drum 103 is turned off(brought to ground potential). At the same, the bias voltage beingimpressed upon the corona discharge unit 111 also is turned off.

The separated transfer paper P is conveyed to the fixing unit 104, wherethe toner images on the transfer paper are fixed, whence the transferpaper is ejected into the paper discharge tray 115.

The foregoing is the printing process in the color LBP used in thisembodiment.

FIG. 1 is a block diagram illustrating an example of the arrangement ofa controller and engine in an image forming apparatus embodying thepresent invention, and FIG. 2 is a block diagram showing a case in whichthe engine of FIG. 1 is evaluated.

The overall configuration will be described first. In FIG. 1, numeral 31denotes the controller and 32 the engine.

The controller 31 includes an RF unit 1 for performing color conversionprocessing such as masking and UCR, a first halftone image processor 2the output of which is supplied to an input terminal b of a switch 4 viaa first density corrector 3, and a first patch pattern generator 5connected to an input terminal a of the switch 4. It is thus possible toswitch between density control and printing.

The output of the switch 4 is supplied to a first modulator 7 via anoutput terminal a of switch 6 provided in the engine 32. The switch hasanother output terminal b connected to a second halftone image processor8, the output of which is supplied to an input terminal b of a switch 10via a second density corrector 9.

A second patch pattern generator 23 is connected to an input terminal aof the switch 10 so that it is possible to switch between densitycontrol and printing. The output of the switch 10 is applied to a secondmodulator 11. The output of the first modulator 7 and the output of thesecond modulator 11 are selectively applied to laser driver 13 by aswitch 12.

Light emitted from a laser 14 is made to scan a photosensitive drum 15by a scanner, not shown, thereby forming a latent image on the drum. Theformed latent image is developed and is transferred either to a transferdrum 16 at the time of density control or to recorded paper, which hasbeen wound upon the transfer drum 16, at the time of printing. Theconnections and arrangements of a density sensor 17, a Dmax processor 19in a CPU 18, a developing high-voltage generator 21 and a developingunit 22 are similar to those of the prior art.

The output of a halftone processor 20 within the CPU 18 is applied tothe second density corrector 9 of the engine 32 and to the first densitycorrector 3 of the controller 31.

In FIG. 2, numeral 41 denotes a test jig for evaluating the engine 32.The test jig 41 is provided with an RF unit 42 made to accommodate theengine process characteristics designated by the engine developer.

Before discussing operation, halftone image processing of the engine 32and automatic density control corresponding to this processing will bedescribed.

As shown in FIG. 2, a designated YMCK signal obtained by the RF unit 42mounted on the test jig 41 is sent to the engine 32 field-sequentiallyas multivalued image data of, say, 200 lines. This signal is applied tothe halftone image processor 8 via the contact b of the switch 6. Thehalftone image processor 8 performs superpixel processing for halftoneprinting of 200 lines, the output of the processor 8 is supplied to themodulator 11 via the density corrector 9 and switch 10, and the outputof the modulator 11 is supplied to the laser driver 13 via the contact bof the switch 12. The modulator 11 is a central-growth pulse-widthmodulator in which growth occurs from the center of pixels incorrespondence with 200 lines of superpixels.

FIG. 3 is a block diagram illustrating an example of the detailedarrangement of the halftone image processor 8, a density corrector 9 andmodulator 11.

Multivalued image data Video 0–7 of, say, eight bits, outputted by thetest jig 41 is latched in a latch 51 synchronized to an image clockVCLK. In a case where simple 200-line printing is carried out, only thislatch suffices for image processing. However, processing such as edgeemphasis, not illustrated, may also be executed.

A gamma-correction RAM 52 outputs image data obtained by subjecting theabove-mentioned image data to a density correction by correction datafrom the halftone processor 20, described later. The image datacorrected for density is latched in a latch 53 synchronized to VCLK.

A clock generator 54 generates a clock signal SCLK having a frequencythat is 512 times that of the image clock VCLK. A counter 55 counts theclock signal SCLK up or down. More specifically, since one input pixelhas 256 gray levels, the counter counts up by 256 and then down by 256in one period of the image clock VCLK. As a result, the output of thecounter is a triangular wave ranging between “00” and “FF”, as shown inthe example of FIG. 4.

A comparator 56 compares the outputs of the latch 53 and counter 55,thereby pulse-width modulating the image data after it isgamma-corrected. The laser driver 13 drives the laser 14 in dependenceupon this pulse-width modulated signal, thereby forming dots which growfrom the centers of pixels so that a halftone image can be reproduced.

Density control will be described next. The density control is executedbefore actual printing takes place. Dmax control will be describedfirst.

Patch data having a prescribed shape created on the basis of aDmax-control patch pattern that has been stored in the patch patterngenerator 23 for the purpose of Dmax control is formed, as a latentimage of a plurality of patch patterns, on the outer peripheral surfaceof the photosensitive drum 15 by the laser 14 through the intermediaryof the contact a of switch 10, the pulse-width modulator 11 and thelaser driver 13.

The Dmax-control patch pattern has several gray levels in the vicinityof maximum density for each color corresponding to the recording paper.

Next, the developing high-voltage generator 21 generates developing biasat a plurality of levels, which vary in stepwise fashion, incorrespondence with the number of patch patterns, whereby theabove-mentioned latent image is visualized by the developing unit 22.This visual image is transferred to the recording paper on the transferdrum 16, and the density of the patch pattern corresponding to thedeveloping bias is measured by the density sensor 17. The Dmax processor19 in the CPU 18 subjects the measurement data to interpolationprocessing, calculates developing bias, at which maximum density isobtained, from the developing-bias—image density characteristicobtained, and sets a developing bias on the basis of the results ofcalculation, thereby holding the maximum density constant.

It should be noted that Dmax control may be adapted to control otherprocess conditions such as corona discharge voltage and fixingtemperature. When the Dmax control is completed, halftone control isexecuted.

FIG. 5 is a diagram illustrating patch formation on the transfer drum16. FIG. 12 is a diagram illustrating the patches formed. Eight patchpatterns 70 having a plurality of density levels for each color, whichpatterns have been generated on the basis of patch patterns for halftonecontrol stored in the patch generator 23, are used to form a latentimage on the photosensitive drum 15 in the same manner as in Dmaxcontrol. However, the latent image is visualized by the developing unit22 while the developing bias is maintained at a prescribed value, andthis image is transferred to the recording paper on the transfer drum16. The transferred plurality of patch patterns each have their colordensity measured by the density sensor 17, and the resulting data isprocessed by the halftone processor 20 within the CPU 18. The halftoneprocessor 20 obtains a density characteristic, an example of which isindicated at 72 in FIG. 6, by interpolation processing based upon theeight items of data. In a case where the obtained density characteristic72 is far removed from a target characteristic 73, the halftoneprocessor 20 calculates correction data by referring to a gamma tablestored in a gamma ROM 24 and stores the correction data in the gammacorrection RAM 52. The output data in the latch 51 is corrected by thiscorrection data. It should be noted that density control performed bythe engine, namely Dmax control and halftone control, is carried out onthe basis of control by the CPU 18 within the engine when the powersupply starts up or periodically, by way of example. Furthermore, thegamma data is data for absorbing a density difference between thedensity detection point (the transfer drum 16) and the actual fixedimage as well as a density difference caused by the environment. Theenvironment mentioned here refers to temperature and/or humidity.Accordingly, a sensor for sensing temperature and/or humidity isprovided within the engine and the output of the sensor is delivered tothe controller.

Printing and density control with regard to halftone image processingperformed by the controller 31 will now be described.

In FIG. 1, any YMCK signal obtained by the RF unit 1 is suppliedfield-sequentially to the halftone image processor 2 as multivaluedimage data of, say, 600 lines. The halftone image processor 2 formssuperpixels by grouping each of the pixels, which are in units of 600lines, every three pixels in the main-scan direction and supplies theseto the density corrector 3 as six-bit image data and two-bit positioninformation pixel by pixel. The output of the density corrector 3 issupplied to the engine 32 via the switch 4. At the time of actualprinting, the switch 4 is connected to the b side and the switch 6 isconnected to the a side. Accordingly, the above-mentioned image data andposition information are supplied to the modulator 7, whence the dataand information are delivered to the laser driver 13 via the contact aof switch 12.

The modulator 7 is a pulse-width modulator which, on the basis of, say,two-bit position information, is capable of selectively subjecting eachpixel of 600 lines to central-growth pulse-width modulation in whichpixels grow from the center thereof, left-growth pulse-width modulationin which pixels grow from the left side, and right-growth pulse-widthmodulation in which pixels grow from the right side.

FIG. 7 is a block diagram illustrating an example of a detailedarrangement of the halftone image processor 2, density corrector 3 andmodulator 7. Switches are not necessary in this description of operationand therefore are not shown.

The multivalued image data Video 0–7 of, say, eight bits from the RFunit 1 is converted to six-bit data and two-bit position informationPos0, Pos1 of each pixel by the halftone image processor 2. The six-bitdata of each pixel is applied to gamma correction RAM 82, and thetwo-bit position information Pos0, Pos1 is applied to a counter 85 inthe engine 32. The gamma correction RAM 82 outputs six-bit image dataobtained by subjecting the entered six-bit data to a density correctionusing correction data from an arithmetic unit 87, described later. Theimage data whose density has been corrected is latched in a latch 83synchronized to VCLK.

A clock generator 84 generates a clock signal SCLK whose frequency is128 times that of the image clock signal VCLK. A counter 85 counts theclock signal SCLK up or down. More specifically, since one input pixelhas 64 gray levels, the counter counts up by 64 and then down by 64 inone period of the image clock VCLK in accordance with the positioninformation signals Pos0, Pos1. As a result, the output value of thecounter is a triangular wave 91 ranging between “00” and “3F”; as shownin the example of FIG. 8. Further, a sawtooth waveform 92 which rises tothe right and a sawtooth waveform 93 which decays to the right can alsobe generated by an up-count every two clocks.

A comparator 86 compares the outputs of the latch 83 and counter 85,thereby pulse-width modulating the image data after it isgamma-corrected. The laser driver 13 drives the laser 14 in dependenceupon this pulse-width modulated signal. As a result, dots which growfrom the centers of pixels, dots which grow from the left side of pixelsor dots which grow from the right side of pixels can be formedselectively depending upon the position information, thus making ispossible to form a halftone.

Density control will be described next. Density control is performed ina manner similar to that set forth above. Eight patch patterns 70, whichhave been generated by the patch generator 5, are formed for each coloron the transfer drum 16 by a sequence already described. Moreover, thepatch pattern data for 600 lines generated by the patch generator 5 istransmitted to the first modulator 7 via the input terminal a of theswitch 4 and the output terminal a of the switch 6, thereby themodulator 7 modulates the patch data. Here there are cases in which thepatch pattern data takes on different values because the halftone imageprocessing is different from that of the method described above. Thepatch patterns each have their color density measured by the densitysensor 17, and the resulting data is processed by the halftone processor20 within the CPU 18. The halftone processor 20 obtains a densitycharacteristic, an example of which is indicated at 72 in FIG. 6, byinterpolation processing based upon the eight items of data. This is setto the density corrector 3 of the controller 31 as density data. Thereare cases in which these characteristics also differ from those of theforegoing method.

In a case where the received density characteristic is far removed fromthe target characteristic, the arithmetic unit 87 calculates correctiondata by referring to a gamma table stored in a gamma ROM 88 and storesthe correction data in the gamma correction RAM 82. The output data ofthe halftone image processor 2 is corrected by this correction data.

In accordance with this embodiment, as described above, the developer ofthe engine initiates automatic density control, which is brought toconclusion within the engine, premised on image processing inside theengine, and performs evaluation based upon a multivalued image signalfrom the RF unit made to correspond to the process characteristics ofthe engine. As a result, it is possible to provide a printer in whichengine quality can be assured and which employs a controller mountingthe designated RF unit.

Further, the developer of the controller is capable of providing thecontroller with functions of its own by performing automatic densitycontrol spanning the controller and the engine, using the densitydetection signal from the engine, on the condition of image processingwithin the controller.

Furthermore, by using the density measurement signal generated by thecontroller and the density information from the engine, the developer ofthe controller may ascertain the process characteristics of the engine,thereby making it possible to raise the degree of freedom and shortendevelopment time.

Further, control of density is not performed merely by a fixed methodbut by using a controller on the basis of a prescribed image patternthat has been subjected to image processing. As a result, image densitycharacteristics to be adjusted are not fixed and image formation can becarried out based upon optimum density characteristics flexiblyconforming to the user needs. This makes it possible to obtain ahigh-quality output image under various environmental condition or inimage processing methods.

[Modification]

An image forming apparatus according to a modification of the firstembodiment will now be described. It should be noted that componentssubstantially similar to those of the first embodiment are designated bylike reference characters and a detailed description thereof is omitted.

FIG. 9 is a block diagram showing an example of an arrangement accordingto a modification of the present invention.

Here the controller 31 includes a first pseudo-halftone processor 101for executing image processing by the dither method, for example, andthe engine 32 includes a second pseudo-halftone processor 102 forexecuting image processing by the error diffusion method, for example.Accordingly, the first density corrector 3 and first modulator 7 areoptimized for the dither method, and the second density corrector 9 andsecond modulator 11 are optimized for the error diffusion method.

By adopting this arrangement, it is possible to provide a printeroptimized for the dither method desired by the controller developer andthe error diffusion method desired by the engine developer.

Further, not only pseudo-halftones but also a binary image may besubjected to smoothing by the controller 31 and edge emphasis by theengine 32. Furthermore, types of image processing inclusive of theprocessing of the first embodiment may be combined freely.

An image forming apparatus according to another modification will now bedescribed. It should be noted that components substantially similar tothose of the first embodiment are designated by like referencecharacters and a detailed description thereof is omitted.

FIG. 10 is a block diagram showing an example of an arrangementaccording to another modification of the present invention. Here thecontroller 31 has a gamma RAM 111 instead of the gamma ROM in order todownload a gamma table from a host computer or the like.

By adopting this arrangement, the controller developer can deal rapidlywith updating of version of the engine 31.

An image forming apparatus according to a modification of the presentinvention will now be described with reference to FIG. 11. It should benoted that components substantially similar to those of the firstembodiment are designated by like reference characters and a detaileddescription thereof is omitted.

FIG. 11 is a block diagram showing an example of an arrangementaccording to the modification. Here the switches 6 and 12 are changedover every pixel by a signal IMCHR 121 showing the attributes of animage.

By adopting this arrangement, data such as character data in whichresolution is of special importance is pulse-width modulated by a600-line modulator, and data such as photographic data in which tonalityis of special importance is pulse-width modulated by a 200-linemodulator, thereby forming an image. As a result, the image quality ofthe overall image can be improved by utilizing two modulatorseffectively.

In the foregoing embodiment and modifications, patch data from a patchgenerator does not undergo corresponding image processing. However, thepatch data can be sent through an image processor as a matter of course.

Further, in accordance the embodiment and modifications described above,the controller also is capable executing image processing of its own. Asa result, a variety of controllers can be made to accommodate one typeof engine so that a greater variety of printers can be used. Inaddition, printing quality can be improved by effectively using twomodulators made to accommodate the image processing of each of thecontroller and engine units.

The present invention can be applied to a system constituted by aplurality of devices or to an apparatus comprising a single device.Furthermore, it goes without saying that the invention is applicablealso to a case where the object of the invention is attained bysupplying a program to a system or apparatus.

In accordance with the first embodiment and modifications thereof, thereare provided an image forming apparatus and method in which a controlleris capable of executing image processing of its own. This makes itpossible to solve the problem of prolonged development time, the problemwherein image processing functions inclusive of color conversion cannotbe added on freely, and the problem wherein engine quality cannot beassured in a case where image processing functions are allowed to beadded on freely.

Second Embodiment

A second embodiment of the invention will now be described withreference to the drawings.

In the second embodiment, components and operations similar to those ofthe first embodiment are designated by like reference characters andneed not be described again.

The following description relates to density control based upon controlperformed by the controller.

In FIG. 14, density control is carried out using video interface linesbetween the controller and the engine, namely a command signal line 27for transmitting eight-bit serial signal in order to designate variousoperations in the engine, a status signal line 25 for receiving aneight-bit serial signal in order to ascertain the status of the engine,an eight-bit parallel data signal line for transmitting print data tothe engine, and a top signal line 26 which the engine uses to requestimage data.

The operating procedure in density control according to this embodimentis illustrated in FIG. 15. First, a CPU 28 within the controller 31sends a density-measurement execution command to the engine 32 via thecommand line when density control is required (S1). After receiving thedensity-measurement execution command, the engine 32 finishes Dmaxcontrol within the engine (S2). When a state is attained in whichdensity measurement becomes possible in response to a request from thecontroller 31, the engine 32 uses the top signal line to request thecontroller 31 to transmit image data (S3). In response, the CPU of thecontroller 31 changes the switch 4 over to the side (a) of the patchpattern generator 5 and sends, say, eight items of patch pattern datafor each color to the engine 32 via the data signal line (S4). Theengine 32 thenceforth forms each of the patch patterns on the transferdrum through a sequence similar to that described above (S5), measuresthe density of each color by the density sensor 17, stores the measureddensities in a memory (not shown) within the halftone processor (S6) andreturns a status signal indicating end of density measurement to thecontroller 31 via the status line (S7).

Upon receiving the status signal indicating end of density measurement,the controller 31 sends the engine 32 a designated density-patch requestcommand via the command signal line in order to ascertain the number ofa patch (one among eight patterns for each color) corresponding todensity data sent from the engine (S8). Upon receiving this command, theengine 32 sends the designated patch number back to the controller 31via the status signal line (S9).

Next, when a density-measurement result request command is sent from thecontroller 31 via the command signal line (S10), the engine 32 reads thedensity data of the designated patch out of the memory and sends backthe data, via the status signal line, as the result of densitymeasurement (S11). After the aforementioned steps S8–S11 have beenrepeated with regard to all patches, the controller 31 obtains a densitycharacteristic, of the kind illustrated at 72 in FIG. 6, from eightitems of data for each color by means of interpolation processing usingthe halftone processor within the CPU, and sends this densitycharacteristic to the density corrector 3 of controller 31 as densitydata (S12).

FIG. 19 shows a block diagram of the CPU 28 of the controller 31 of FIG.14. The functions of the CPU 28 may be implemented, for example, usingsoftware programs executed in a central processing unit that isconventionally and generally widely known in the field of the inventionand to those skilled in the art.

It should be noted that density control in the engine 32 is periodicallyperformed, which is independent from the above-mentioned density controlin the controller 31.

That is, an image forming at 200 lines which is automatically controlledby the engine 32 guarantees the same color forming as that when theapparatus is shipped. On the other hand, an image forming at 600 linesis controlled by the controller 31, thus providing a color formingdepending on a usage of the image.

Thus, in accordance with the second embodiment as set forth above,communication of information relating to density control can be carriedout accurately between the engine and the controller using the statussignal, top signal and command signal. As a result, density control canbe performed from the controller side.

(Modification)

An example in which the timing of density control in the secondembodiment is judged by the controller upon receiving the status fro theengine will be described as a modification with reference to FIG. 16.

FIG. 16 is a block diagram showing an example of the arrangement of thecontroller and engine in an image forming apparatus according to thismodification of the invention.

A wake-up signal from a wake-up sensor 45, a door-open signal from adoor-open sensor 46 and a toner signal, which indicates whether or notthere is any toner, from a remaining-toner sensor 47 are connected toports of the CPU 18. Along with a signal 48 indicating completion ofDmax control within the engine and a signal 49 indicting completion ofhalftone control within the engine, these signals are sent to thecontroller 31 as status signals via a status processor 44.

When the printing operation is not in effect, the controller 31 sends adensity-control execute status request command to the engine 32 via acommand line at a prescribed period. In response to this command, theengine 32 sends a density-control execute status signal back to thecontroller 31 via the status line. This status signal indicatespower-on, door open, remaining amount of toner, completion of Dmaxcontrol within the engine and completion of halftone control within theengine, etc.

The controller 31 senses wake-up (restoration from a sleep mode) or dooropen (as when jamming occurs), which are factors causing anenvironmental change within the engine. If the amount of remaining toneris adequate, the controller 31 shifts to density-control execution.

Further, when it is sensed that there is inadequate toner, thecontroller 31 does not execute density control because such controlcannot be performed accurately under such conditions.

If it so happens that the current time is a moment immediately afterexecution of Dmax control in the engine, then Dmax control from thecontroller 31 is skipped and control starts from halftone control.

If it so happens that the current time is a moment immediately afterexecution of halftone control, the number of patches formed at detectionof density is reduced so that the time needed for density control can becurtailed.

According to this embodiment, the signal indicating completion of Dmaxcontrol and the signal indicating completion of halftone control areused as control status signals within the engine. However, a signalindicating that execution of Dmax control is in progress or a signalindicating that execution of halftone control is in progress may be usedtogether with these signals.

In accordance with this embodiment, as described above, wake-up sensingmeans, a door-open sensing means and residual-toner sensing means, whichis used to determine whether performing density control is meaningful ornot, are employed to sense a change in the status of an engine for whichthere is a possibility that density control is necessary. Along withthese signals, signals indicating status of density control execution bythe engine itself, namely a signal indicating completion of Dmax controlwithin the engine and a signal indicating completion of halftone controlwithin the engine are sent to a controller. As a result, the controlleris capable of appropriately judging the timing at which density controlshould be executed, thus making highly precise control of densitypossible.

Further, waiting time until printing begins can be shortened because theforegoing need not be performed redundantly with regard to densitycontrol requests from both the controller and engine.

Further, S8–11 shown in FIG. 15 may be converted to a step of issuing acommand for designating a density patch number, a step of issuing acommand requesting the results of density measurement, and a step ofreceiving, in the form of a status signal, results of densitymeasurement of the designated density patch number.

Further, an arrangement may be adopted in which the controllerdetermines whether Dmax control is to be performed or not. Specifically,in the command (S1) for executing measurement of density in FIG. 15, anarrangement may be adopted in which it is possible to designate whetherDmax control is to be performed or not. A case in which Dmax control ispassed may start from S4.

Further, an arrangement may be adopted in which whether or not Dmaxcontrol is to be performed is judged by the controller based upon astatus signal from the engine. Specifically, an arrangement may beadopted in which when Dmax control was performed is indicated by astatus signal, with the controller making the judgment based uponelapsed time from the previous Dmax control event.

Further, though the formed patches shown in FIG. 12 are formed for everycolor, it is permissible to form them for every halftone, as shown inFIG. 17.

Further, though an LBP is employed in the foregoing embodiments, thisdoes not impose a limitation upon the present invention. For example, itis permissible to use an image processing apparatus in which an image isformed by employing a head of the type in which film boiling is producedby thermal energy so as to jet droplets of ink.

Further, the controller in each of the foregoing embodiments may residein a host device such as an external apparatus.

Furthermore, in case of forming an image at 200 lines, an input signalmay be processed to pass through the halftone image processer 2 and thedensity corrector 3 in the controller 31.

An arrangement may also be adopted in which each of the halftone imageprocesser 2 and the density corrector 3 provides a halftone imageprocessing at 200 and 600 lines and a density correction for 200 and 600lines. In this arrangement, the halftone image processing and thedensity correction for 200 lines and these processing and correction for600 lines are switched over properly. Moreover, in this case, it mayalso be arranged that a density control for both 600 lines and 200 linesin accordance with a control of the controller 31 by holding the patchdata for 600 and 200 lines in the patch generator 5.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

1. A controller for outputting color image data to an image formingunit, said controller comprising: a communicator, arranged to performbi-directional communication with the image forming unit; a statusreceiver, arranged to receive a status of the image forming unit byusing said communicator; a decider, arranged to decide timing forcalibrating a condition of color processing based on the receivedstatus; a transmitter, arranged to transmit a command, which indicatesexecution of calibration of the condition of color processing, to theimage forming unit by using said communicator; a data receiver, arrangedto receive patch data, obtained by measuring one a patch formed by theimage forming unit, from the image forming unit by using saidcommunicator; a calculator, arranged to calculate controllercolor-correction data based on the received patch data; and corrector,arranged to perform color correction using the controllercolor-correction data so as to generate color image data to be outputtedto said image forming unit, wherein the image forming unit has a centralprocessing unit which controls the formation and measurement of thepatch in accordance with the command received from said transmitter. 2.A controller according to claim 1, wherein the status of the imageforming unit includes power-on, open, and remaining amount of tonerstatus indications.
 3. A controller according to claim 1, wherein thestatus of the image forming unit includes a remaining amount of tonerstatus indication, and wherein calibration is not executed when theremaining amount of toner is less than a predetermined amount.
 4. Animage forming unit comprising a central processing unit executing amethod comprising the steps of: performing bi-directional communicationwith a controller; receiving image data from the controller to form acolor image by using the bi-directional communication; transmitting astatus of the image forming unit to the controller by usingbi-directional communication; receiving from the controller a command,which indicates execution of a calibration of a condition of colorprocessing, by using the bi-directional communication; controllingformation and measurement of a patch by the image forming unit inaccordance with the received command; and transmitting patch dataobtained in said controlling step to the controller by using thebi-directional communication, wherein the controller calculatescontroller color-correction data based on the patch data and performscolor correction using the controller color-correction data so as togenerate color image data to be outputted to the image forming unit. 5.A controlling method of a controller comprising the steps of: performingbi-directional communication with the image forming unit; receiving astatus of the image forming unit from the image forming unit by usingthe bi-directional communication; deciding timing for a calibratingcondition of color processing based on the received status; transmittingto the image forming unit a command, which indicates execution ofcalibration of the condition of color processing, by using thebi-directional communication; receiving from the image forming unitpatch data, which is obtained by measuring a patch formed by the imageforming unit, by using the bi-directional communication; calculatingcontroller color-correction data based on the received patch data; andperforming color correction using the controller color-correction dataso as to generate color image data to be outputted to the image formingunit, wherein the image forming unit has a central processing unit whichcontrols the formation and measurement of the patch in accordance withthe command received in said transmitting step.